Multilayer circuit boards (MLBs) permit formation of multiple circuits in a minimum volume or space. These typically comprise a stack of layers of signal, ground and/or power planes (lines) separated from each other by a layer of dielectric material. The lines are often in electrical contact with each other by plated holes passing through the dielectric layers. The plated holes are often referred to as “vias”.
Known processes for fabricating MLBs typically comprise fabrication of separate innerlayer circuits, which are formed by coating a photosensitive layer or film over a copper layer of a copper clad innerlayer base material. The photosensitive coating is imaged, developed and etched to form conductor lines. After etching, the photosensitive film is stripped from the copper leaving the circuit pattern on the surface of the innerlayer base material.
After formation of individual innerlayer circuits, a multilayer stack is formed by preparing a lay-up of innerlayers, ground planes, power planes, etc., typically separated from each other by a dielectric pre-preg typically comprising a layer of glass cloth impregnated with partially cured material, typically a B-stage epoxy resin. The top and bottom outer layers of the stack comprise copper clad, glass filled, epoxy planar boards with the copper cladding comprising exterior surfaces of the stack. The stack is laminated to form a monolithic structure using heat and pressure to fully cure the B-stage resin. The stack so formed typically has copper cladding on both of its exterior surfaces. Exterior circuit layers are formed in the copper cladding using procedures similar to the procedures used to form the innerlayer circuits. A photosensitive film is applied to the copper cladding. The coating is exposed to patterned activating radiation and developed. An etchant is then used to remove copper bared by the development of the photosensitive film. Finally, the remaining photosensitive film is removed to provide the exterior circuit layers.
Conductive vias (or interconnects) are used to electrically connect individual circuit layers within an MLB to each other and to the outer surfaces and typically pass through all or a portion of the stack. Vias are generally formed prior to the formation of circuits on the exterior surfaces by drilling holes through the stack at appropriate locations. Following several pre-treatment steps, the walls of the vias are catalyzed by contact with a plating catalyst and metallized, typically by contact with an electroless or electrolytic copper plating solution to form conductive pathways between circuit layers. Following formation of the vias, exterior circuits, or outerlayers are formed using the procedure described above.
Following MLB construction, chips and other electrical components are mounted at appropriate locations on the exterior circuit layers of the multilayer stack, typically using solder mount pads to bond the components to the MLB. The components are in electrical contact with the circuits within the MLB through the conductive vias. The pads are typically formed by coating an organic solder mask coating over the exterior circuit layers. The solder mask may be applied by screen coating a liquid solder mask coating material over the surface of the exterior circuit layers using a screen having openings defining areas where solder mount pads are to be formed. Alternatively, a photoimageable solder mask may be coated onto the board and exposed and developed to yield an array of openings defining the pads. The openings are then coated with solder using procedures known to the art such as wave soldering.
Complexity of MLBs has increased significantly over the past few years. For example, boards for mainframe computers may have as many as 36 layers of circuitry or more, with the complete stack having a thickness of about 0.250 inch. These boards are typically designed with three or five mil wide signal lines and twelve mil diameter vias. For increased densification in many of today's MLBs, the industry desires to reduce signal lines to a width of two mils or less and vias to a diameter of two mils or less. Most known commercial procedures, especially those of the nature described hereinabove, are incapable of economically forming the dimensions desired by the industry.
In addition to decreasing line width and via diameter, the industry also desires to avoid manufacturing problems frequently associated with MLB manufacture. As described above, current procedures utilize innerlayer materials that are glass-reinforced resin or other suitable dielectric material layers having a thickness of from about four to five mils clad with copper on both surfaces. The glass reinforcing material is used to contribute strength and rigidity to the MLB stack. However, since lamination is typically at a temperature above 150° C., the resinous portion of the laminate shrinks during cooling to the extent permitted by the rigid copper cladding. If the copper is etched to form a discontinuous pattern, laminate shrinkage may not be restrained by the copper cladding. This problem is exacerbated as feature size decreases. Consequently, further shrinkage may occur. The shrinkage may have an adverse affect on dimensional stability and registration between board layers.
The first step to form a MLB involves lay-up of layers prior to lamination. Care must be exercised to avoid shifting of the innerlayers during lamination. Otherwise, the layers will not be aligned and electrical contact between layers will not be achieved. In addition, during lay-up, air may be trapped in spaces adjacent signal lines because a solid pre-preg is laid over the signal lines that does not completely fill all recesses between signal lines. Care must be taken to evacuate such entrapped air. Residual air pockets can cause defects and subsequent problems during use of the multilayer board.
The use of glass reinforced inner and outerlayer materials creates additional problems. The glass fiber is needed for board strength. However, when holes are drilled to form vias, glass fibers can extend into the holes and, if so, must be removed prior to metallization. Removal creates the need for additional pretreatment steps such as the use of glass etchants to remove glass fibrils extending into the holes. If the glass is not removed, a loss of continuity might occur in the metal deposit. In addition, the glass fibers add weight and thickness to the overall MLB. Materials which do not require reinforced glass fiber (or as much as previous materials) or the like have also been developed to overcome this particular problem.
One improvement in the manufacture of MLBs is disclosed in U.S. Pat. No. 5,246,817. In accordance with the procedures of the '817 patent, manufacture of the MLB comprises sequential formation of layers using photosensitive dielectric coatings and selective metal deposition procedures. In accordance with the process of the patent, the first layer of the board is formed over a temporary or permanent carrier that may become an integral part of the board. When the carrier is a circuit, the process comprises formation of a dielectric coating over the circuit with imaged openings defining the vias. The imaged openings may be obtained by exposure of a photosensitive dielectric coating to activating radiation through a mask in an imaged pattern followed by development to form the imaged openings. Alternatively, imaging may be by laser ablation in which case, the dielectric material need not be photosensitive. Metal is deposited into the recesses within the dielectric coating to form vias. Thereafter, an additional layer of dielectric is coated onto the first dielectric layer, imaged in a pattern of circuit lines, and the recesses are then plated with metal. Alternatively, after imaging the first dielectric coating, it may be coated with a second dielectric coating and imaged and the recesses plated with metal to form the vias and circuit lines simultaneously. By either process, the walls of the imaged opening or recesses in the dielectric coating contain metal as it deposits during plating and assures a desired cross-sectional shape of the deposit. Plating desirably fills the entire recess within the imaged photosensitive coating. The process is repeated sequentially to form sequential layers of circuits and vias.
The method disclosed in this patent include alternative selective metal plating methods whereby metal is selectively deposited within the imaged openings to render the same conductive. The procedures disclosed in the patent involve selectively depositing metal in imaged openings without increase in the surface resistivity of an underlying substrate between conductor lines. Selective metal deposition may be performed by several new techniques disclosed in the patent to avoid increased conductivity between signal lines. The selective deposition procedures of the '817 patent typically involve multiple coating steps using sacrificial layers.
Other examples of methods of making circuitized substrates such as MLBs are described and illustrated in the following documents:
  6,506,979Shelnut et alU.S. 2002/0150673Thorn et alU.S. 2002/0170827FuruyaU.S. 2002/0172019Suzuki et alU.S. 2002/0190378Hsu et alU.S. 2003/0022013Japp et al
As described herein, the present invention represents a significant improvement over processes such as those above in the production of circuitized substrates such as MLBs. One particularly significant feature of this invention is the provision of conductive material within the product's vias (openings) using a commoning bar or layer such that two and more consecutive openings can be provided with the necessary conductive material to perform the necessary connective functions required of such vias. Another feature is the provision of such conductive material without the need for a “seed layer” during at least part of the product's formation; such seed layers are often required in many plating operations for MLBs and related products having conductive circuitry as part thereof.
It is believed that such an invention will represent a significant advancement in the art.